JP2019521368A - HHG source, inspection apparatus, and method of performing measurements - Google Patents

Abstract

A method for performing measurements with an inspection device and associated inspection devices and HHG sources are disclosed. The method sets one or more controllable characteristics of at least one drive laser pulse of the harmonic generating radiation source to control the output radiation spectrum of the illumination radiation provided by the harmonic generating radiation source. Illuminating the target structure with the illumination radiation. The method can include setting the drive laser pulse such that the output radiation spectrum includes a plurality of discrete harmonic peaks. Alternatively, the method can include using multiple drive laser pulses of various wavelengths such that the output emission spectrum is substantially monochromatic. [Selection] Figure 4

Description

Cross-reference to related applications
[0001] This application claims the priority of European Patent Application Publication No. 16167512.9 filed on April 28, 2016, which is hereby incorporated by reference in its entirety. .

  FIELD [0002] The present invention relates to HHH sources, inspection apparatus, and methods of performing measurements. In particular, the present invention relates to an inspection apparatus included in a lithographic apparatus and a method of performing measurements in the inspection apparatus.

  A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). At that time, patterning devices, alternatively referred to as masks or reticles, can be used to generate circuit patterns to be formed on the individual layers of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one or more dies) of a substrate (e.g. a silicon wafer). Multiple layers, each with a specific pattern and material composition, are added to form interconnects of functional devices and finished products.

  [0004] In lithographic processes, it is often desirable to make measurements of the formed structure, for example, to provide process control and verification. Various tools are known to perform the above measurements, including scanning electron microscopes often used to measure critical dimensions (CDs), alignment accuracy of the two layers of the device, and dedicated tools to measure overlays. . Recently, various forms of scatterometers have been developed for use in the lithography field.

  [0005] In the known scatterometer example, it is often necessary to provide a dedicated metrology target. For example, the method may require a target in the form of a simple grid, which in the form of a grid causes the measurement beam to form a smaller spot than the grid (ie the grid is underfilled ) Enough big enough. In the so-called reproduction method, the characteristics of the lattice can be calculated by simulating the interaction of scattered radiation using a mathematical model of the target structure. The parameters of the model are adjusted until the simulated interaction results in a diffraction pattern similar to that observed from the actual target.

  [0006] In addition to measuring feature shapes by reproduction, overlays can be measured based on diffraction using the apparatus described in US Patent Application Publication No. 2006066855 A1. Diffraction-based overlay metrology, which uses a dark-field image of the diffraction orders, allows measurement of overlays of smaller targets. These targets can be smaller than the illumination spots and can be surrounded by product structures on the wafer. Examples of dark field imaging metrology can be found in a number of published patent applications, such as, for example, U.S. Patent Application Publication No. 2011102753A1 and U.S. Patent Application Publication No. 20120044470A. Multigrids can be measured in one image using a composite grid target. Known scatterometers tend to use light in the visible or near-infrared region, and light in this region has a grating pitch that is much coarser than the actual product structure whose characteristics are actually of interest. I need. Such product features can be identified using much shorter wavelength deep ultraviolet (DUV) or extreme ultraviolet (EUV) radiation. Unfortunately, such wavelengths can not usually be used or used for metrology.

  [0007] On the other hand, the dimensions of modern product structures are so small that they can not be imaged by optical metrology techniques. Small features include, for example, those formed by multi-patterning processes and / or pitch multiplication. Thus, targets used for high-volume metrology often use features that are much larger than products whose overlay errors or critical dimensions are of interest. The measurement results are only indirectly related to the dimensions of the actual product structure, the metrology target is not subjected to the same distortion under light projection in the lithographic apparatus and / or at other steps of the manufacturing process It may not be accurate because it does not receive another treatment. While scanning electron microscopy (SEM) can resolve these modern product structures directly, SEM takes much more time than optical measurements. Furthermore, electrons can not penetrate the thick process layer, which makes them less suitable for metrology applications. Although other techniques are also known, such as measuring electrical properties using contact pads, this technique presents only indirect evidence of true product structure.

  [0008] By shortening the wavelength of the radiation used during metrology (ie, moving it towards the soft x-ray wavelength spectrum), the radiation can resolve smaller structures, ie, against structural variations of the structure As the sensitivity is higher, measurement performance is improved. However, this requires a corresponding improvement of the spectral resolution of the metrology system. Furthermore, the complexity of the product structure is increasing, and the product structure is increasing in number of layers and correspondingly thick. Furthermore, this increases the spectral resolution needed to perform the metrology measurements.

  [0009] In a first aspect of the present invention, one or more of at least one drive laser pulses of a harmonic generating radiation source is provided to control an output radiation spectrum of illumination radiation supplied by the harmonic generating radiation source. A method is provided for performing measurements in an inspection apparatus, including setting controllable characteristics and illuminating a target structure with the illumination radiation.

  [0010] The invention further provides a test apparatus comprising a harmonic generating radiation source, operable to perform the method of the first aspect.

  The present invention further provides an inspection apparatus including a harmonic generation radiation source, the harmonic generation radiation source including a drive laser source operable to emit a drive laser pulse, the drive laser pulse of The one or more controllable characteristics are set such that the output radiation spectrum of the harmonic generating radiation source comprises a plurality of discrete harmonic peaks.

  The present invention further provides an inspection apparatus including a harmonic generation radiation source, wherein the harmonic generation radiation source is operable to emit driving laser pulses each having a different central wavelength. Includes a laser source.

  The present invention further provides a harmonic generation radiation source comprising a plurality of drive laser sources operable to emit drive laser pulses each having a different center wavelength.

  The present invention further provides a harmonic generating radiation source including a driving laser source operable to emit a driving laser pulse, wherein one or more controllable characteristics of the driving laser pulse are harmonic generation The output radiation spectrum of the radiation source is set to include a plurality of discrete harmonic peaks.

  [0015] In the above, it should be understood that "a plurality of driving laser sources" includes a plurality of independent laser devices or (more generally) a single laser device whose output is split into multiple beams Some or all of the multiple beams may be adjusted in some way (e.g., frequency converted by a frequency converter element) to change the wavelength of the beams. Thus, in this context, the drive laser source can be, for example, the laser device itself and / or a frequency converter element.

  The invention further provides a computer program product comprising one or more consecutive machine readable instructions for performing the setting step of the method according to the first aspect.

  [0017] Further aspects, features, and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art from the teachings set forth herein.

  Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which like reference numerals indicate like parts.

A lithographic apparatus is shown together with other apparatus that form a manufacturing facility for semiconductor devices. Fig. 3 shows the geometry of the incident and reflected light rays on a grating target in a metrology method according to a first embodiment of the invention. Fig. 3 schematically shows the components of the metrology apparatus implementing the method of Fig. 2 (a). Fig. 1 schematically shows an inspection apparatus suitable for implementing the method according to an embodiment of the present invention. Fig. 4 schematically illustrates the configuration of a radiation source used in the device of Fig. 2 (b) or Fig. 3; FIG. 5 is a graph of intensity (y-axis) versus energy (x-axis) showing the emission spectrum of the HHG source of an embodiment of the present invention. FIG. 7 is a graph of intensity (y-axis) versus time (x-axis) showing the laser driven pulses of the HHG source of an embodiment of the present invention. FIG. 7 is a graph of intensity (y-axis) versus energy (x-axis) showing the emission spectrum of a HHG source driven by a laser with three different central drive wavelengths. FIG. 7 is a graph of first order diffraction angle (y-axis) versus wavelength (x-axis) for normal incident radiation on a 40 nm pitch grating, showing the effect of wavelength at scattering.

  Before describing the embodiments of the present invention in detail, it is useful to present an exemplary environment in which the embodiments of the present invention may be practiced.

  FIG. 1 shows at 200 a lithographic apparatus LA as part of a manufacturing facility for performing a mass lithographic manufacturing process. In this example, the manufacturing process is suitable for manufacturing semiconductor products (integrated circuits) on a substrate such as a semiconductor wafer. Those skilled in the art will appreciate that variations of this process and processing of various types of substrates can produce a wide variety of products. The production of semiconductor products of great commercial importance today is used as an example only.

  In the lithographic apparatus (or “litho tool” 200 for short) 200, the measurement station MEA is shown at 202 and the exposure station EXP is shown at 204. The control unit LACU is shown at 206. In this example, each substrate stays at a measurement station and an exposure station having a pattern to be applied. In an optical lithography apparatus, the projection system is used, for example, to transfer a product pattern from the patterning device MA to a substrate using a coordinated radiation and projection system. This is done by forming a pattern image on the layer of radiation sensitive resist material.

  [0022] The term "projection system" as used herein is refractive, which is suitable to use exposure radiation or to other elements, such as the use of immersion liquid or the use of a vacuum. It should be broadly interpreted to encompass any type of projection system, including reflective, catadioptric, magnetic, electromagnetic and electrostatic optics, or any combination thereof. The patterning MA device may be a mask or reticle, and the pattern or reticle imparts a pattern to a radiation beam transmitted or reflected by the patterning device. Known operating modes include stepping mode and scanning mode. As is known, the projection system can cooperate in various ways with the support and positioning system for the substrate and patterning device to add the desired pattern to multiple target portions across the substrate. A programmable patterning device can be used in place of a reticle having a fixed pattern. The radiation may include, for example, electromagnetic radiation in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) frequency band. The present disclosure is also applicable to other types of lithographic processes, such as by electron beam, eg, imprint lithography and direct writing lithography.

  [0023] Lithographic apparatus control unit LACU receives substrate W and reticle MA and controls all operations and measurements of the various actuators and sensors in order to carry out the patterning operation. The LACU also has signal processing and data processing capabilities to perform the desired calculations associated with the operation of the device. In practice, the control unit LACU is embodied as a system consisting of a number of subunits, each handling real-time data acquisition, processing and control of subsystems or components within the device.

  The substrate is processed at the measurement station MEA before the pattern is applied to the substrate at the exposure station EXP so that various preliminary steps can be performed. The preliminary step can include mapping the surface height of the substrate using a height sensor, and measuring the position of the alignment mark on the substrate using an alignment sensor. The alignment marks are usually arranged in a regular grid pattern. However, due to inaccuracies in forming the marks, the marks also deviate from the ideal grid due to substrate deformation that occurs throughout processing of the substrate. Because of this, in addition to measuring the position and orientation of the substrate when the device prints product features to precise positions with extremely high accuracy, in practice the alignment sensor actually marks a large number of marks across the substrate area The position of the must be measured in detail. The apparatus can be of the so-called two-stage type, with two substrate tables each containing a positioning system controlled by a control unit LACU. While one substrate mounted on one substrate table is exposed at the exposure station EXP, another substrate can be mounted on the other substrate table at the measurement station MEA so that various preliminary steps can be performed . Therefore, the measurement of the alignment mark is very time consuming, and providing two substrate tables enables a significant improvement of the throughput of the device. While the substrate table is at the measurement station and the exposure station, a second position sensor is provided which enables the position sensor IF to detect the position of the substrate table at both stations if the position of the substrate table can not be measured. be able to. The lithographic apparatus LA may be, for example, a so-called two-stage type having two substrate tables WTa, WTb and two stations (exposure station and measurement station) at which the substrate tables can be exchanged.

  [0025] Within the manufacturing facility, apparatus 200 is also a "litho cell" or "litho cluster" that also contains a coating apparatus 208 that applies a photosensitive resist and other coatings to substrate W to form a pattern with apparatus 200. Form a part. At the output side of the apparatus 200, a baking apparatus 210 and a developing apparatus 212 are provided to develop the exposed pattern into a physical resist pattern. Between all these devices, the substrate handling system takes care of the support of the substrate and the transfer of the substrate from one device to the next. These devices, often collectively referred to as tracks, are under the control of a track control unit, which itself is controlled by a supervisory control system (SCS) 238, which also controls lithography. Control the lithographic apparatus through the device control unit LACU. Thus, various devices can operate to maximize throughput and processing efficiency. The supervisory control system SCS receives recipe information R which defines in more detail the definition of the steps to be carried out to form each patterned substrate.

  [0026] Once the pattern is applied in the lithographic cell and developed, the patterned substrate 220 is transferred to other processing devices, such as indicated at 222, 224, 226, and so forth. Various processing steps are performed by the various devices of a typical manufacturing facility. As an example, the device 222 of this embodiment is an etch station, and the device 224 performs a post etch annealing step. Additional physical and / or chemical processing steps may be applied to additional devices 226 and the like. Various types of processing, such as deposition of materials, modification of surface material properties (oxidation, doping, ion implantation, etc.), chemical mechanical polishing (CMP), etc. may be required to fabricate actual devices. Device 226 may, in fact, represent a series of various processing steps performed on one or more devices.

  [0027] As is known, the fabrication of semiconductor devices requires multiple repetitions of such processing in order to build the device structure using the appropriate materials and patterns for each layer of the substrate. Correspondingly, the substrate 230 which has reached the litho cluster can be a freshly prepared substrate, or the substrate 230 can be a substrate already processed in this cluster or in a completely different apparatus. . Similarly, depending on the processing required, the substrate 232 can be returned to the same litho cluster for the next patterning process upon exiting the device 226, or different destinations for the patterning process. Or can be a finished product sent for dicing and packaging.

  [0028] Each layer of the product structure requires a different set of process steps, and the devices 226 used in each layer may be completely different in type. Furthermore, even if the processing steps applied by the device 226 are nominally the same, in large installations, it may be considered several identical working in parallel to perform the step 226 for various substrates There can be a machine. Small differences in configuration or defects between these machines can mean that the small differences affect the various substrates in different ways. Even steps that are relatively common to each layer, such as etch (apparatus 222), may be performed by several etchers that are nominally identical but operate in parallel to maximize throughput. In fact, furthermore, the various layers require different etching processes, for example chemical etching, plasma etching, depending on the details of the material to be etched and the special requirements, such as, for example, anisotropic etching. .

  [0029] As mentioned above, the previous and / or subsequent processes may be performed on other lithographic apparatus, and may even be performed on various types of lithographic apparatus. For example, some layers that are highly demanding with regard to parameters such as resolution and overlay can be implemented with more advanced lithography tools in the device manufacturing process than other layers that are less demanding. Thus, some layers can be exposed with an immersion type lithography tool, while other layers are exposed with a "dry" tool. Some layers can be exposed with tools operating at DUV wavelengths, while other layers are exposed using radiation at EUV wavelengths.

  [0030] In order for the substrate exposed by the lithographic apparatus to be exposed accurately and consistently, the exposed substrate is inspected to characterize the overlay error between the substrate layers, line thickness, critical dimension (CD), etc. It is desirable to measure Correspondingly, the manufacturing facility in which the lysocell LC is arranged also includes the metrology system MET, which receives some or all of the substrates W processed in the lysocell. Metrology results are supplied directly or indirectly to the supervisory control system SCS. If an error is detected, the exposure of the next substrate can be adjusted, especially if the metrology can be done as quickly and quickly as other substrates of the same batch are still exposed. Also, already exposed substrates can be stripped and reprocessed to improve yield, or discarded, thereby avoiding performing further processing on substrates that are known to be defective. If only a portion of the target portion of the substrate is defective, further exposure can be performed only on the target portion that is good.

  Also shown in FIG. 1 is a metrology device 240 provided to measure product parameters at desired stages of the manufacturing process. A common example of a metrology apparatus in a state-of-the-art lithography manufacturing facility is a scatterometer, for example, an angle resolved scatterometer or a spectroscopic scatterometer, which is used prior to etching in the apparatus 222. It can be used to measure the properties of the substrate developed at 220. The metrology device 240 can be used to reveal that important performance parameters, such as, for example, overlays or critical dimensions (CDs), do not meet specific accuracy requirements with the developed resist. Prior to the etching step, there is an opportunity to strip the developed resist and reprocess the substrate 220 by lithoclusters. As is also known, the metrology results 242 from the apparatus 240 are used to maintain high accuracy performance of the patterning process on the litho cluster and to make minor adjustments over time by means of the supervisory control system SCS and / or the control unit LACU 206. To minimize the risk of the product falling out of specification or requiring reprocessing. It will be appreciated that the metrology device 240 and / or other metrology devices (not shown) can be used to measure the properties of the processed substrates 232, 234 and the input substrate 230.

  [0032] Each generation of lithographic fabrication techniques (commonly referred to as the technique "node") has strict specifications for performance parameters such as CD. One of the major issues of metrology is that the size of the metrology target is desirably smaller than the targets typically used by the metrology device 240. For example, the current goal is to use a 5 μm × 5 μm or smaller target. These small sizes are the so-called "in-die" metrology in which the target is placed in the product structure (instead of being confined to the scribe lane area between the product areas), or the target is the product structure itself Enables a wider use of “on product” metrology. The only metrology technology currently used for on-product CD metrology is electron microscopy (CD-SEM). This known technique shows limits for the next generation of nodes and only provides very limited geometry information of the structure.

  [0033] One approach to improving the metrology of minimal structures is to use shorter wavelength radiation, for example, extreme ultraviolet (EUV), soft x-rays, and even hard x-ray regions. For example, the EUV reflectometer side including spectral EUV reflection measurement is considered to be a CD metrology method for the next generation technology node. In transmission mode (T-SAXS) or grazing incidence mode (GI-SAXS), x-ray scattering techniques such as small angle x-ray scattering are also contemplated. The principles and implementation of EUV metrology associated with this are presented in the above-mentioned patent application EP 15 160 786. There, it is clearly shown that the EUV reflectometer side offers the benefit of high sensitivity, is robust to process variations and is selective to the parameters of interest.

  [0034] In the present disclosure, hard x-rays are considered to be light beams having a wavelength of less than about 0.1 nm, including, for example, the range of 0.01 to 0.1 nm. Soft x-ray or EUV refers to a range spanning wavelengths of about 0.1 nm to 125 nm. Various sub-ranges of these ranges can be selected according to the dimensions of the structure to be inspected. For example, for semiconductor structures that are at the limit of current lithographic techniques, wavelengths in the range of 0.1 to 20 nm, or 0.1 to 10 nm, or 1 to 5 nm are contemplated. Not only the size of the structures but also their material properties can influence the choice of wavelengths used in the inspection. For example, to perform reflection measurements, at least the background material of the structure needs a good reflection intensity at the wavelengths used. For inspection of buried features, the wavelength must be selected to penetrate the overlying material sufficiently.

  [0035] EUV metrology (after development inspection or ADI) measures the structure in the resist material processed in the lithocell and / or (after etching inspection or AEI) the structure is more rigid Once formed of the material, it can be used to measure the structure. For example, the substrate may be inspected using EUV metrology device 244 after being processed in development apparatus 212, etching apparatus 222, annealing apparatus 224, and / or other apparatus 226.

  [0036] For mass production applications, high intensity radiation sources are desirable to reduce the acquisition time of each measurement. The limited power of current compact x-ray sources means that the throughput of the known T-SAXS technology is very low, especially for small size metrology targets. This is especially the case when trying to obtain a very small spot diameter in order to illuminate a small target area on the substrate. The known EUV sources also have limited brightness and limited choice of wavelength. Precise control of the wavelength over a wide range is desirable to maximize the contrast of the target structure and to distinguish structures of different materials.

  The manufacturing system shown in FIG. 1 includes one or more EUV metrology devices 244 in addition to the optical scatterometer 240. The EUV metrology apparatus provides additional metrology results 246, which are used by the supervisory control system SCS to provide further control of the overall quality and performance improvement of the lithographic manufacturing system. be able to. Similar to the optical scatterometer 240, the metrology device 244 can be used at various stages of manufacture, such as ADI and AEI described above.

EUV reflectometer side method
FIG. 2 shows (a) a metrology method and (b) a metrology device 300. The apparatus may be used to measure parameters of the substrate W processed by the manufacturing system of FIG. 1, as an example of an EUV metrology device 244. The device can be used in wavebands other than EUV.

  [0039] In FIG. 2 (a), the target T is schematically illustrated as including a one-dimensional lattice structure at the origin of the spherical reference frame. The axes X, Y, Z are defined relative to the target. (Of course, in principle any coordinate system can be set, each component can have its own local reference frame that can be set for what is shown). The direction of the periodicity D of the target structure is coincident with the X axis. The drawings are not true perspective views, they are only schematically illustrated. The X-Y plane is the plane of the target and the substrate, which is shown tilted with respect to the viewer for clarity, and is illustrated by the oblique view of the circle 302. The Z direction defines a direction N perpendicular to the substrate. In FIG. 2 (a), the incident radiation is labeled 304 and has a grazing incidence angle α. In this example, the incident beam 304 (and all incident beams forming the radiation spot S) are defined in the directions D, N and a plane substantially parallel to the XZ plane, which is the plane represented by the circle 306 Located in Reflected rays 308 (ie specular rays) that are not scattered by the periodic structure of the target T exit at the height angle α towards the right hand side of the target in the figure.

  The other rays 310 scatter at an angle different from specular reflection, according to the diffractive properties of the target. The separation angle between these rays and specular rays is determined by the relationship between the wavelength of the radiation and the spacing of the features of the target. The drawings are not to scale. For example, the detector 312 may be closer to or further from the target than shown, the target grating is often very small compared to the detector, and the diffraction angle of the beam 310 is as illustrated Much wider than.

  The light beam 308 and / or the scattered light beam 310 are captured by the light detector 312 in order to perform a reflection measurement. Detector 312 includes, for example, a position sensitive EUV detector, typically an array of detector elements. The array may be a linear array, but in practice a two dimensional array of elements (pixels) may be provided. The detector 313 can be, for example, a CCD (charge coupled device) image sensor or a CMOS image sensor. This detector is used to convert the reflected radiation into an electrical signal and ultimately into digital data for analysis. In principle, a single pixel detector may be sufficient to make several types of measurements. Having a two-dimensional image detector allows more flexible operation.

  From the measured spectra obtained for one or more wavelengths and one or more incident angle values α, it is possible to calculate measured values of the properties of the target structure T in the manner further described below.

EUV reflection measurement device
[0043] Referring to FIG. 2 (b), a metrology apparatus 300 for measuring the characteristics of the metrology target T formed on the substrate W by the method of FIG. 2 (a) is presented. Various hardware components are shown schematically. The practical implementation of these components can be performed by those skilled in the art using a mixture of existing components and specially designed components based on known design principles. A support (not shown in detail) is provided to hold the substrate in the desired position and orientation relative to the other components described. Radiation source 330 provides radiation to illumination system 332. The illumination system 332 provides the radiation beam indicated by the light beam 304, which (with the other light beams forms the illumination beam) forms a focused illumination spot on the target T. Detector 312 and any optical components can be conveniently considered as detection system 333.

  The substrate W in this example is attached to a movable support having a positioning system 334 so that the angle of incidence λ of the light beam 304 can be adjusted. In this example, tilting the substrate W is conveniently selected to change the angle of incidence while the radiation source 330 and the illumination system 332 remain stationary. In order to capture the reflected beam 308, the detection system 333 is provided with a further movable support 336 so that the detection system 333 moves over the angle 2α with respect to the stationary illumination system or the angle α with respect to the substrate Move across. In the oblique incidence regime of reflection measurement, it is convenient to define the incident angle α with reference to the plane of the substrate as shown. It will be appreciated that the angle of incidence α can also be defined as the angle between the direction of incidence of the incident beam 304 and the direction N perpendicular to the substrate.

  [0045] In an alternative embodiment, the angle of incidence can be varied in more than one dimension, for example by using a conical mount. This type of device and its potential benefits are described in detail in the above-mentioned European patent application no. 15160786. The entire content of that application is incorporated herein by reference.

  [0046] An additional actuator, not shown, is provided to carry each target T to the position where the focusing spot S of the radiation is located. (If you think about this in another way, take the spot to the position where the target was placed). In practical applications, there can be a series of individual targets to be measured or the position of one target on a single substrate, and there can be a series of substrates. In principle, whether the substrate and the target can be moved and turned while the illumination system and the detector remain stationary, or if the substrate remains stationary while the illumination system and the detector move. It does not matter if the relative movement of the various components is performed by a combination of these techniques. The present disclosure includes all of these variations.

  The radiation reflected from the target T and the substrate W is split into spectra 310 of rays of various wavelengths before striking the detector 312, as already described with reference to FIG. 2 (a). A second detection 338 is also typically provided to measure the intensity of the incident beam and use it as a reference. Processor 340 receives signals from detectors 312, 338. The reflection data obtained for one or more angles of incidence are used by the processor to calculate a measurement of the property of the target, for example a CD or an overlay.

  [0048] FIG. 3 (a) schematically shows the main elements of an inspection apparatus that implements so-called dark field imaging metrology, in particular in the context of overlay metrology. The apparatus may be a stand-alone device or may be incorporated into the lithographic apparatus 200, for example, at either the measurement station 202 or the metrology stations 240, 244. The optical axis with several branches throughout the device is indicated by the dotted line O. The target grating structure T and the diffracted rays are shown in more detail in FIG. 3 (b).

  [0049] As described in the prior application described in the introduction section, the dark field imaging apparatus of FIG. 3 (a) may be used in place of or in addition to a spectroscopic scatterometer. Can be part of a multi-purpose angle-resolved scatterometer. In this type of inspection device, the radiation emitted from the radiation source 11 is conditioned by the illumination system 12. For example, the illumination system 12 can include a collimating lens system and an aperture device. The conditioned radiation follows the illumination light path, is reflected by the partially reflective surface 15 in the illumination light path and is focused on the spot S on the substrate W via the microscope objective 16. The metrology target T can be formed on the substrate W. The lens 16 preferably has a high numerical aperture (NA) of at least 0.9, more preferably at least 0.95. If necessary, immersion fluid can be used to obtain numerical apertures greater than one. Multi-purpose scatterometers can have more than one measurement branch. Furthermore, practical devices include further optics and branches for collecting reference radiation, eg for intensity normalization, for coarse imaging of acquisition targets, for focusing etc. These details can be found in the above mentioned prior publication. In the present disclosure, only the measurement branches targeted for dark field imaging metrology are shown and described in detail.

  In the dark field imaging collection path, the imaging optics 21 form an image of the target on the substrate W on a sensor 23 (eg, a CCD or CMOS sensor). The aperture stop 20 is provided on the plane P 'of the collection passage. The plane P ′ is a plane conjugate to the pupil plane P (not shown) of the objective lens 16. The aperture stop 20 may be referred to as a pupil stop. The aperture stop 20 can take various forms, as can the illumination openings take various forms. The aperture stop 20 cooperates with the effective aperture of the lens 16 to define which part of the scattered radiation is used to form an image on the sensor 23. Typically, the aperture stop 20 functions to block the zero order diffracted beam so that the image of the target formed on the sensor 23 is formed only from the primary beam. In the example where both primary beams are combined to form an image, this image is a so-called dark-field image corresponding to a dark-field microscope. However, in the present application, only one primary is imaged at a time, as described below. The image captured by the sensor 23 is output to an image processor and controller 40, these functions depending on the particular type of measurement to be made. For this application, a measurement of the asymmetry of the target structure is made. Asymmetry measurements can be combined with target structure information to obtain measurements of performance parameters of the lithographic process used to form the target structure. Performance parameters that can be measured in this manner may include, for example, overlay, focal length, and dose.

  [0051] If a metrology target T is provided on the substrate W, the metrology target T can be a 1D grid, and the 1D grid is printed after development so that the bars are formed by continuous resist lines Ru. The target can be a 2D grid, which after development is printed to be formed with successive resist pillars or vias in the resist. Alternatively, the bars, pillars or vias can be etched into the substrate. Each of these grids is an example of a target structure that can be characterized using an inspection device.

  [0052] The various components of the lighting system 12 are adjustable to perform various metrology "recipes" within the same device. In addition to selecting wavelength (color) and polarization as particular properties, the illumination system 12 can be adjusted to implement various illumination profiles. Since the plane P ′ ′ is conjugate to the pupil plane P of the objective lens 16 and the plane of the detector 19, the illumination profile of the plane P ′ ′ defines the angular distribution of light incident on the spot S on the substrate W. An aperture device can be provided in the illumination path to implement different illumination profiles. The aperture device can include various apertures attached to the moveable slider or wheel. Alternatively, the illumination path can include a programmable spatial light modulator. As a further alternative, optical fibers can be placed at various locations in plane P ′ ′ and used to selectively transmit or not transmit light to the respective locations of the optical fibers. Variants are described and illustrated in the above-mentioned document.

  [0053] In the illumination mode of the first example, the light beam 30a is supplied such that the incident angle is as shown by "I", and the optical path of the zero-order light beam reflected by the target T is denoted as "0" (Do not mix with the optical axis "O"). In a second illumination mode, a light beam 30b can be provided, in which case the angles of incidence and reflection are switched. Both of these illumination modes are recognized as off-axis illumination modes. Many different illumination modes can be implemented for various purposes.

  As shown in more detail in FIG. 3 (b), the target grating T as an example of the target structure is arranged such that the substrate W is perpendicular to the optical axis O of the objective lens 16. In the case of an off-axis illumination profile, the rays of the illumination I which strike the grating T at an angle off the axis O are the zero-order ray (solid line 0) and the two primary rays (dashed-dot line +1 and dash-dot line -1 A). If the small target grid is overfilled, it should be remembered that these rays are only one of many parallel rays across the area of the substrate that contains the metrology grating T and other features. Because the beam of illumination beam 30a has a finite width (needed to accommodate useful light), incident beam I virtually occupies a predetermined angular range, and diffracted beam 0 and diffracted beam + 1 /- 1 spreads somewhat. According to the small target point spread function, each order +1, -1 is not a single ideal ray as shown, but extends further over a predetermined angular range.

  With further reference to FIG. 3 (a), under the first illumination mode using ray 30a, the + first order diffracted ray from the target grating enters objective lens 16 and the image is recorded on sensor 23 Contribute to being If the second mode of illumination is used, ray 30b will be incident at an angle opposite to that of ray 30a so that the -first order diffracted ray will enter the objective and contribute to the image. The aperture stop 20 blocks zero-order radiation when using off-axis illumination. As described in the prior publication, illumination modes can be defined with off-axis illumination in the X and Y directions.

  Under these various illumination modes, asymmetry measurements can be made by comparing the images of the target gratings. Alternatively, the asymmetry measurement can be performed by maintaining the same illumination mode but rotating the target. Although off-axis illumination is shown, alternatively, on-axis illumination of the target can be used, using the modified off-axis aperture 20 to send substantially only one first order diffracted light to the sensor be able to. In a further example, prisms are used instead of the aperture stop 20 and these prisms have the effect of deflecting the + 1st and -1st orders to different positions of the sensor 23 so that there is no need for two successive image capture steps , +1 and -1 orders can be detected and compared. This technology is disclosed in the above-mentioned published patent application US Patent Application Publication No. 2011102753 A1, the contents of which are incorporated herein by reference. Instead of or in addition to the primary beam, secondary, tertiary and higher order beams (not shown in FIG. 3) can be used for the measurement. As a further variation, the off-axis illumination mode can be kept constant, while the target itself is rotated 180 ° under the objective 16 to work the image using the opposite diffraction order.

  [0057] Some scatterometry techniques are usually performed using radiation having visible wavelengths. Thus, a scatterometry target (or a target on which at least higher order diffraction measurements are made) has a pitch that is greater than the pitch of the product structure on the substrate. As an example, a scatterometry target can have a target grating pitch in microns (μm), whereas product structures on the same substrate can have a pitch in nanometers (nm).

  [0058] This pitch difference results in an offset between the measured overlay of the product structure and the actual overlay. This offset is at least partially due to light projection distortion in the lithographic apparatus and / or another treatment in another step of the manufacturing process. Currently, offsets are largely responsible for the overall measured overlay. Thus, reducing or eliminating this involvement improves the overall overlay accuracy.

  [0059] In order to carry out diffraction-based measurements on structures using a pitch corresponding to the pitch of the product structure, it is necessary to use radiation of shorter wavelength than visible light. However, some materials currently used in many layers of product structures, such as polycrystalline silicon or amorphous carbon, absorb ultraviolet light. On the other hand, it has been found that the absorption loss for the radiation of "soft x-ray" spectrum (about 2 nm to 40 nm) is much smaller than for ultraviolet light. It will be appreciated, however, that the specific absorbance of the product structure depends on the particular material used in the layer. Based on the information on the properties of the material used in a particular structure, it is possible to select the emission wavelength to be used to minimize absorption losses.

[0060] For example, in order to maximize the accuracy of diffraction-based measurements to determine overlay errors, it is necessary to optimize the characteristics of the radiation reaching the detector. The properties of the scattered radiation depend on the properties of the radiation used and on the properties of the structure to be measured. One parameter that can be used to describe the quality of scattered radiation is the so-called "stack sensitivity". This parameter represents the strength of the measured asymmetry caused by the upper target grid shifting relative to the lower target grid. The "stack sensitivity" has been found to change periodically depending on the wavelength of the radiation and the thickness of the target structure. The period of change Δλ _s that defines the resolution with respect to the stack thickness T can be expressed by the following equation.

[0061] where λ is the wavelength of the radiation and T is the optical thickness of the structure to be measured. An exemplary optical thickness of the product structure can be 400 nm and an exemplary emission wavelength can be λ = 13 nm. In this example, the period Δλ _s of the change in “stack sensitivity” is 0.21 nm.

[0062] In order to optimize the radiation measured at the detector, it is necessary for the inspection apparatus to have a better spectral resolution than the size of the cyclic change Δλ _s of the stack sensitivity. In particular, to fully resolve periodic changes in stack sensitivity, the required spectral resolution Δλ _r of the inspection system should be at least twice the resolution of the fluctuation period Δλ _s . Thus, in this example, the required spectral resolution Δλ _r of the inspection apparatus can be about 0.1 nm.

The spectral resolution of an inspection apparatus such as that shown in FIG. 2 (b) or FIG. 3 is determined by the characteristics of the optical system and the characteristics of the target structure. Due to target size limitations, the spot diameter of a typical inspection device is limited to about 2 μm. Assuming that the illumination radiation is a Gaussian beam, the following relationship between the beam waist diameter D and the numerical aperture NA (half angle) of the illumination radiation can be derived.

  For illumination radiation having a wavelength of λ = 13 nm, the numerical aperture can be derived as NA = 4 mrad.

Currently, the pitch of the product structure is approximately P = 40 nm. The spectral resolution (ie, grating resolution) Δλ _g of a diffraction-based inspection apparatus (eg, scatterometer) that measures target structures having this pitch is given by the following equation (assuming λ << pitch size): It can be derived as the street.
Δλ _g 22 × P × NA = 80 × 0.004 = 0.32 nm. (3)

The grating resolution Δλ _g is larger than the required spectral resolution Δλ _{r of} 0.1 nm. In other words, periodic changes in stack sensitivity can not be resolved sufficiently. The lattice resolution Δλ _g can be improved by reducing the size of the numerical aperture. However, this results in the need to increase the target size. This is because the spot diameter increases as the NA decreases. As mentioned above, the target must be "underfilled" (ie the spot diameter is smaller than the size of the target). Thus, as the spot diameter increases, the target must also increase proportionately. The larger the target, the more space it occupies on the surface of the substrate, which is undesirable in terms of manufacturing environment as it increases the cost of manufacturing per product. In the following, a method and an apparatus for improving the spectral resolution of an inspection device are described.

[0067] The HHG (harmonic generation) source is preferred to generate the wavelength needed to measure the product resolution pitch because of the high luminance. Such HHG sources allow small focused spots to be located on diffraction based overlay (DBO) targets with high photon flux. However, the output of the HHG is not a single spectral line, but instead the output of the HHG can include discrete harmonic orders or be a supercontinuum of soft x-ray wavelengths. The accuracy of such DBO techniques depends on good spectrally resolved measurements, ie Δλ _g <= Δλ _r <Δλ _s . Monochromators can be used but the losses are very high in the soft x-ray region. Furthermore, the monochromator adds bulk and limits the flexibility of the measurement setup.

  [0068] FIG. 4 shows a radiation source 402 that includes, for example, a generator of EUV radiation based on harmonic generation (HHG) technology. The main components of the radiation source are the pump laser 420 and the HHG gas cell 422. The gas supply unit 424 supplies an appropriate gas to the gas cell. The pump laser can be, for example, a Ti: sapphire-based laser that generates pulses in the subpicosecond region, having a wavelength of 800 nm with a repetition rate of several kHz. Alternatively, the pump laser can be a fiber laser with an optical amplifier, optionally with a pulse repetition rate up to several MHz. Typical pulse durations can be in the subpicosecond range. The wavelength can be, for example, around 1 μm. The laser pulses are sent to the HHG gas cell 422 as a first radiation beam 428 where a portion of the radiation is converted to a higher frequency. Output radiation beam 430 includes coherent radiation at one or more desired EUV wavelengths. One or more filter devices 432 can be provided. For example, a filter such as thin film aluminum (Al) can function to block the basic IR radiation from going further into the inspection device. It should be noted that some or all of the radiation paths can be contained within a vacuum environment, and the desired EUV radiation is absorbed as it travels through the air. The various components of the radiation source 402 and the illumination optics 404 are adjustable to perform various metrology "recipes" within the same apparatus. For example, different wavelengths and / or polarizations can be selectable.

  [0069] It is proposed to set the HHG emission spectrum to optimize the spectral resolution and / or maximize the power of a particular wavelength. In a first embodiment, it is proposed to set one or more controllable characteristics of the driving laser pulse of the HHG to obtain an HHG emission spectrum comprising narrow bandwidth harmonic peaks, the harmonic peak One or more of the have bandwidths that meet the spectral resolution requirements described above. Thus, this bandwidth (as measured full width at half maximum) can be less than 0.2 nm, less than 0.15 nm, or less than 0.12 nm. In embodiments, this bandwidth may be around 0.1 nm. The drive pulses can also be set so that the spacing between adjacent harmonic peaks is large enough to resolve spectral blurring due to the small NA and grating pitch requirements as shown in equation (3).

  [0070] FIG. 5 shows a schematic HHH emission spectrum 500, which contrasts the intensity along the y-axis with the energy along the x-axis (harmonic order). The bandwidth (denoted a in FIG. 5) of the single harmonic 510 is usually related to the spectral bandwidth of the drive laser. The narrower the spectral bandwidth of the drive laser (ie, the longer the laser drive pulse is in time), the thinner each harmonic peak. This is because a typical drive laser field contains multi-cycle oscillations, and each cycle can generate harmonics. The final HHG emission spectrum is a coherent superposition of everything that occurs. The higher the averaging cycle, the steeper the harmonics.

  [0071] FIG. 6 shows the laser drive pulse 600 as a graph of intensity (arbitrary units) vs. time along the x-axis (fs (femtoseconds)) along the y-axis. Only a relatively small number of cycles (in the laser drive pulse 600 that can generate HHG in the cut-off region of the emission spectrum 500 (orders with relatively high photon energy, eg, orders 71-81) That is, there is a cycle (in the rectangle 610). Thus, the bandwidth of these harmonics is relatively wide due to the low average intensity. However, harmonics in the plateau region of the HHG spectrum 500 with relatively low photon energy are generated by a large number of cycles (ie, cycles within the square 620) and as a result usually have a very steep spectral line width .

  Therefore, it is proposed to obtain steep harmonic peaks (and thus high spectral resolution) by using relatively long laser drive pulses and selecting harmonics in the plateau region of HHG spectrum 500. . In this context "selection" can include the use of the generated harmonics for a particular measurement. This can include configuring the system so that the required diffraction order from the selected harmonic hits the detector and measurements are taken at this harmonic. However, there is a practical limit on the duration of the laser drive pulse. The peak intensity is required to be high enough to enter the tunnel ionization region. In addition, pulses that are too long in duration can cause ground state ionization depletion, so that there are no electrons left for HHG. Thus, it is proposed that the laser drive pulse comprises 10-30 cycles (resulting in a duration of about 25-75 fs) or 15-30 cycles (duration of about 37-75 fs). In a more limited embodiment, the laser drive pulse has about 15 to 25 cycles (duration of about 37 to 63 fs), or about 17 to 22 cycles (duration of about 43 to 55 fs), or about 19 or 20 cycles ( The duration can include about 50 fs).

上式で、λ_Ｄは、レーザ駆動パルスの波長であり、λは、高調波Ｎ（偶数の高調波のみが存在する）に対応する（ＥＵＶ）波長である。このスペクトル間隔Δλ_ｈは、式（１）の変動周期Δλ_ｓと同様にλ^２に比例する。 The spectral interval Δλ _h (denoted as b in FIG. 5) between adjacent harmonic orders is represented by the following equation.

Where λ _D is the wavelength of the laser drive pulse and λ is the (EUV) wavelength corresponding to harmonic N (only even harmonics are present). This spectrum interval Δλ _h is proportional to λ ² , similarly to the fluctuation period Δλ _s of equation (1).

  Another controllable characteristic of a laser drive pulse is its central wavelength. The spectral position of the HHG emission spectrum changes with the central wavelength of the laser drive pulse, so the position of a particular harmonic can be spectrally adjusted by appropriate adjustment of the central wavelength of the laser drive pulse. This can be used to optimize the measurement sensitivity, as it correlates to the measurement radiation wavelength. In embodiments, this is such that the selected harmonic (eg, a harmonic in the plateau region of the emission spectrum) has a peak intensity corresponding to the desired wavelength, eg, the wavelength of maximum sensitivity for measurement. Adjusting the central wavelength of the laser drive pulse.

  FIG. 7 shows a first emission spectrum 700 (dotted line) corresponding to a first laser drive pulse center wavelength (eg, 790 nm) and a first emission spectrum 700 corresponding to a second laser drive pulse center wavelength (eg, 795 nm) Intensity (y-axis) versus output wavelength (x-axis) showing a second emission spectrum 710 (dotted line) corresponding to a second emission spectrum 710 (solid line) and a third laser drive pulse center wavelength (e.g. 800 nm) Is a graph of The bottom graph shows the spectrum over a series of wavelengths along the x-axis, corresponding to the intensity along the y-axis (log scale). The top graph shows the details of the bottom graph in the region of the corresponding single harmonic of each spectrum (note the linear scale of the y-axis which makes the peaks appear wider in the top graph). The wavelength of this single harmonic may be varied by appropriate control of the center wavelength of the laser drive pulse (about 12.79 nm to 12.96 nm in this particular example).

  [0076] Other systems, such as optical parameter amplification (OPA) systems (for example, using second harmonic generation) have a wide adjustment range for the center wavelength of the laser drive pulse, for example, an adjustment range of 580 nm to 2600 nm. As enabled, such a system can provide additional adjustability.

  It is further seen from FIG. 7 that the bandwidth for each harmonic peak is about 0.1 nm for a laser drive pulse of about 50 fs (19 to 20 cycles), where 0.1 nm is Shorter than the desired spectral resolution for overlay sensitivity.

上式で、ｐは格子ピッチサイズ（例えば、ｐ＝４０ｎｍ）であり、λは放射波長である。 [0078] As an alternative to increasing the spectral resolution of the multi-wavelength HHG source output, another embodiment proposes to provide an adjustable monochromatic HHG output. Monochromatic sources make measurement easier in practice. In embodiments, it is proposed to adjust the monochromatic HHG emission such that the monochromatic HHG emission corresponds to the maximum overlay sensitivity and / or CD sensitivity to the target. Monochromators can be used for this purpose, but they have very high losses in the soft x-ray region. Furthermore, the monochromator adds bulk and limits the flexibility of the measurement setup. Here, an optical method of gating out the required spectral region of HHG emission is described. The adjustable feature of this technique can also be used to increase photon resolution by bringing the output wavelength closer to the pitch size of the target. According to the first order grating equation for the normal incident line, the diffraction angle θ is expressed by the following equation.

Where p is the grating pitch size (eg, p = 40 nm) and λ is the emission wavelength.

FIG. 8 is a graph showing the relationship between the diffraction angle θ (y axis) and the wavelength λ (x axis) for a grating with a pitch p = 40 nm. As can be seen, the scattering is considerably larger for wavelengths close to the pitch size p as compared to the scattering of the narrower wavelength. Equation (3), which represents the relationship between the grating resolution Δλ _g , NA, and the pitch, is basically a linearization of Equation (5) applied to the full aperture angle 2NA of the beam, within the restriction λ << p. It is. If small wavelength limitation is not taken, the relationship is expressed by the following equation.

It is now clear that an optimal resolution Δλ _g can be obtained by bringing the wavelength λ closer to the pitch p. For example, determining the value of this equation for a wavelength of about 13 nm and an NA of 0.004 yields a grating resolution Δλ _g = 0.3 nm, which is the required resolution Δλ _r = 0.1 nm Greater than. Therefore, this dispersion is not sufficient to resolve the high speed oscillation of the stack sensitivity. However, if the wavelength of HHG is adjusted to a wavelength λ of about 38 nm, a grating resolution Δλ _g = 0.1 nm is obtained, which is sufficient to resolve the fast oscillations of the stack sensitivity. Also, since equation (1) is proportional to the wavelength, the longer the wavelength, the larger the required resolution Δλ _r can be, and thus wavelengths shorter than 38 nm already provide sufficient resolution.

  [0081] It is proposed to temporarily adapt the full electrical driving field so that the HHG emission spectrum is enhanced in the narrow wavelength band but suppressed in the other bands. This can be done by combining multiple laser fields, each with a different center wavelength. Such a concept is described in the article "Control of bandwidth and enhanced wavelength of an ultraviolet spectrum generated in shaped laser field", Zhang C. et al; Optics Express Vol. 20, No. 15. In a specific non-limiting example, it is proposed to generate each field using 400 nm (second harmonic generation SHG), 800 nm and 2000 nm (OPA) driven laser pulses. Compared to the technique taught by Zhang, these wavelengths proved more practical since two OPAs are not required. The 400 nm pulse can be obtained by doubling the frequency of the 800 nm direct laser output (e.g., using a frequency converter element such as [beta] -barium borate (BBO) crystal). In such embodiments, only 2000 nm pulses are obtained using OPA.

  [0082] A narrow emission spectrum is realized (for example, only a single harmonic is preferable and a quasi-monochromatic output is obtained) by adjusting one or more controllable characteristics of drive laser pulses of different wavelengths. It is proposed to optimize the HHG emission spectrum by In this regard, the quasi-monochromatic output can include an output at which the intensity of peak harmonic radiation is at least an order of magnitude greater than the other harmonic radiation of the HHG emission spectrum. The controllable features can include one or more of intensity, pulse duration, and / or carrier envelope phase. After this optimization is performed, the intensity ratio between one of the drive laser pulses and one or two of the other drive laser pulses is used to select the actual spectral region output (e.g., output peak wavelength) It can be changed. In a particular embodiment, the intensity ratio may be that of the 2000 nm drive laser pulse to the sum of the 400 nm and 800 nm drive laser pulses, which intensity ratio is to change the drive field and thus the radiation spectrum. Changed to select a region.

  [0083] Using the proposed method, in a broad spectral range of less than 10 nm (eg, 2 nm) to 40 nm, or 10 nm to 15 nm (eg, at least an order of magnitude greater than other harmonics of the HHG emission spectrum Peak HHG harmonic emissions can be adjusted.

  [0084] The adjustable HHG source described herein provides great freedom for EUV overlay and CD measurement. The EUV radiation source can be freely adapted to optimize EUV overlay performance according to different targets with different stack thickness, reflectivity, pitch size. The choice of wavelength is a trade-off between various aspects such as, for example, the degree of transmission (transparency of the interlayer), the optical contrast between the grating material and the surrounding material, and the spectral resolution (eg wavelength close to the pitch) .

[0085] Further embodiments according to the present invention are presented in the following numbered clauses.
1. A method of measuring with an inspection device,
Setting one or more controllable characteristics of at least one drive laser pulse of the harmonic generating radiation source to control the output radiation spectrum of the illumination radiation supplied by the harmonic generating radiation source;
Illuminating the target structure with the illumination radiation;
Method including.
2. The method according to clause 1, wherein the setting step comprises setting one or more controllable characteristics of the drive laser pulse such that the output radiation spectrum comprises a plurality of discrete harmonic peaks.
3. The method of clause 2, wherein the bandwidth of each harmonic peak is narrower than 0.2 nm.
4. The method according to clause 2 or 3, wherein the spectral spacing between adjacent harmonic peaks is greater than the bandwidth of the harmonic peaks.
5. 4. The method according to any of the clauses 2-4, wherein the setting step comprises setting a central wavelength of the drive laser pulse to control at least one wavelength of a harmonic peak.
6. Clauses 2-5, wherein the setting step comprises setting the center wavelength of the drive laser pulse to optimize at least one wavelength of harmonics in order to maximize the measurement sensitivity of the target structure. The method according to any one of the preceding claims.
7. The output radiation spectrum includes a plateau region having a plurality of the above-mentioned harmonic peaks with similar intensities, and a cutoff region in which the intensity largely falls for each successive harmonic peak, and the wavelength is controlled / optimized harmonics The method according to clause 5 or 6, wherein the at least one of the wave peaks is a harmonic peak in the plateau region.
8. Method according to any one of the preceding clauses, wherein the intensity of the drive laser pulse is a long cycle and the setting step comprises setting the number of cycles of the drive laser pulse to be greater than 15 .
9. 8. Method according to clause 8, wherein the setting step comprises setting the number of cycles of the drive laser pulse to 15-30.
10. The setting step includes setting a driving field of the harmonic generating radiation source by setting one or more controllable characteristics of a plurality of driving laser pulses including at least partially different central wavelengths. The method described in clause 1.
11. 11. The method of clause 10, wherein the drive laser pulse comprises three.
12. The setting step temporarily adapts the drive electric field of the harmonic generating radiation source, thereby enhancing the narrow band of the output radiation spectrum and suppressing the remaining part of the output radiation spectrum. Clause 10. The method according to clause 10 or 11, comprising setting the one or more controllable characteristics of the drive laser pulse.
13. 13. A method according to clause 12, wherein the narrow band enhancement is such that the output radiation spectrum is substantially monochromatic.
14. The intensity of one of the drive laser pulses to the other one or more of the drive laser pulses is varied to control the peak wavelength of said narrow band of the enhanced output emission spectrum, clause 12 or 13. Method described.
15. The intensity of one of the drive laser pulses to the other one or more of the drive laser pulses is the peak wavelength of the narrow band of the enhanced output emission spectrum to maximize the measurement sensitivity of the target structure. The method according to any one of clauses 12, 13 or 14, wherein the method is changed to optimize.
16. Clause 12. The setting step includes setting one or more controllable characteristics of the plurality of drive laser pulses such that the narrow band peak wavelength of the enhanced output emission spectrum is 2 nm to 40 nm. 15. The method according to any one of 15.
17. Clause 12. The setting step includes setting one or more controllable characteristics of the plurality of drive laser pulses such that the narrow band peak wavelength of the enhanced output emission spectrum is 9 nm to 15 nm. The method according to any one of -16.
18. One of the plurality of drive laser pulses is obtained directly from the drive laser output, and at least one other of the plurality of drive laser pulses is obtained by converting the drive laser output using a frequency converter element. The method according to any one of clauses 10-17.
19. 22. The method according to clause 18, wherein another one of the plurality of drive laser pulses is obtained by optical parameter amplification.
20. Detecting scattered radiation from said illumination of the target structure;
Determining overlay offsets between the various layers of the target structure from the scattered radiation;
A method according to any one of the preceding clauses, including
21. An inspection apparatus comprising a harmonic generating radiation source and operable to perform the method according to any one of the preceding clauses.
22. 20. A computer program product comprising machine readable instructions which, when run on a suitable processor, cause the processor to perform at least the setting steps of the method according to any one of the clauses 1-20.
23. An inspection apparatus comprising a harmonic generating radiation source, the harmonic generating radiation source comprising a driving laser source operable to emit driving laser pulses;
An inspection apparatus, wherein one or more controllable characteristics of the drive laser pulse are set such that the output radiation spectrum of the harmonic generating radiation source comprises a plurality of discrete harmonic peaks.
24. 24. An inspection apparatus according to clause 23, wherein the bandwidth of each harmonic peak is narrower than 0.2 nm.
25. 25. An inspection apparatus according to clause 23 or 24, wherein the spectral spacing between adjacent harmonic peaks is greater than the bandwidth of the harmonic peaks.
26. 23. Inspection apparatus according to any of clauses 23-25, wherein a central wavelength of the drive laser pulse is configurable to control at least one wavelength of a harmonic peak.
27. The central wavelength of the drive laser pulse can be set to optimize at least one wavelength of the harmonics in order to maximize the measurement sensitivity of the measured target structure, any one of clauses 23 to 26 The inspection apparatus described in the section.
28. The output radiation spectrum includes a plateau region having a plurality of the above-mentioned harmonic peaks with similar intensities, and a cutoff region in which the intensity largely falls for each successive harmonic peak, and the wavelength is controlled / optimized harmonics 26. An inspection apparatus according to clause 26 or 27, wherein the at least one of the wave peaks is a plateau region harmonic peak.
29. 23. Inspection apparatus according to any of clauses 23-28, wherein the intensity of the drive laser pulse is a long lasting cycle, and the number of cycles of the drive laser pulse is greater than 15.
30. 33. Inspection apparatus according to clause 29, wherein the number of cycles of the drive laser pulse is 15-30.
31. An inspection apparatus comprising a harmonic generating radiation source, the harmonic generation radiation source comprising a plurality of driving laser sources operable to emit driving laser pulses each having a different center wavelength.
32. 33. An inspection apparatus according to clause 31, wherein the drive laser source comprises three.
33. One or more controllable characteristics of each drive laser pulse temporarily adapt the drive electric field of the harmonic generation radiation source, thereby enhancing the narrow band of the output radiation spectrum of the harmonic generation radiation source, 33. Inspection apparatus according to clause 31 or 32, wherein the remaining part of the output radiation spectrum is set to be suppressed.
34. 33. The inspection apparatus according to clause 33, wherein the one or more controllable characteristics of each drive laser pulse are set such that the narrowband enhancement causes the output radiation spectrum to be substantially monochromatic.
35. Clause 33, or the intensity of one of the drive laser pulses to one or more of the other of the drive laser pulses is controllable to control the peak wavelength of said narrow band of the enhanced output emission spectrum The inspection apparatus of 34.
36. The intensity of one of the drive laser pulses to the other one or more of the drive laser pulses is the peak wavelength of the narrow band of the enhanced output emission spectrum to maximize the measurement sensitivity of the measured target structure. The inspection device according to any one of clauses 33, 34 or 35, wherein the inspection device is configurable to optimize.
37. 33. Inspection apparatus according to any of clauses 33 to 36, wherein a plurality of driving laser sources are set such that the narrow band peak wavelength of the enhanced output radiation spectrum is less than 20 nm.
38. 33. Inspection apparatus according to any of clauses 33 to 36, wherein a plurality of drive laser sources are set such that the peak wavelength of the narrow band of the enhanced output emission spectrum is around 40 nm.
39. The plurality of drive laser sources is provided from a single laser device, so that one of the drive laser sources is obtained directly from the output of the laser device, and at least one other of the drive laser sources has a frequency of 31. An inspection apparatus according to any one of clauses 31 to 38 obtained by converting a laser device output using a converter element.
40. 39. Inspection apparatus according to clause 39, wherein said further driven laser source is obtained by optical parameter amplification of the laser device output.
41. A harmonic generation radiation source comprising a plurality of drive laser sources operable to emit drive laser pulses each having a different center wavelength.
42. 42. The harmonically generated radiation source according to clause 41, wherein said driving laser source comprises three.
43. One or more controllable characteristics of each drive laser pulse temporarily adapt the drive electric field of the harmonic generation radiation source, thereby enhancing the narrow band of the output radiation spectrum of the harmonic generation radiation source, 43. A harmonic generating radiation source according to clause 41 or 42, configured to suppress the remaining part of the output radiation spectrum.
44. 43. A harmonic generating radiation source according to clause 43, wherein the one or more controllable characteristics of each drive laser pulse are set such that the narrowing of the narrow band enhancement causes the output radiation spectrum to be substantially monochromatic.
45. Clause 43, or the intensity of one of the drive laser pulses to one or more of the other of the drive laser pulses is configurable to control the peak wavelength of the narrow band of the enhanced output emission spectrum 44. The harmonic generation radiation source as described in 44.
46. For an inspection apparatus, the intensity of one of the drive laser pulses to one or more of the other of the drive laser pulses is enhanced to maximize the measurement sensitivity of the measured target structure. 43. A harmonic generating radiation source according to any one of clauses 43, 44 or 45, wherein the harmonic generating radiation source is configurable to optimize the narrow band peak wavelength.
47. 42. A harmonic generating radiation source according to any of clauses 43 to 46, wherein a plurality of driving laser sources are set such that the narrow band peak wavelength of the enhanced output radiation spectrum is less than 20 nm.
48. 42. A harmonic generating radiation source according to any of clauses 43 to 46, wherein a plurality of driving laser sources are set such that the peak wavelength of the narrow band of the enhanced output radiation spectrum is around 40 nm.
49. The plurality of drive laser sources are derived from a single laser device, so that one of the drive laser sources is obtained directly from the output of the laser device, and at least one other of the drive laser sources is 42. A harmonic generating radiation source according to any one of clauses 41 to 48, obtained by converting a laser device output using a frequency converter element.
50. Another harmonic generating radiation source according to clause 49, wherein said driving laser source is obtained by optical parameter amplification of the laser device output.
51. A harmonic generating radiation source comprising a driving laser source operable to emit a driving laser pulse,
One or more controllable characteristics of the drive laser pulse are harmonic generation radiation sources, wherein the output radiation spectrum of the harmonic generation radiation source is set to include a plurality of discrete harmonic peaks.
52. The harmonic-generating radiation source according to clause 51, wherein the bandwidth of each harmonic peak is narrower than 0.2 nm.
53. 52. A harmonic generating radiation source according to clause 51 or 52, wherein the spectral spacing between adjacent harmonic peaks is greater than the bandwidth of the harmonic peaks.
54. 51. A harmonic generating radiation source according to any of clauses 51 to 53, wherein a central wavelength of the drive laser pulse is configurable to control at least one wavelength of a harmonic peak.
55. The center wavelength of the drive laser pulse can be set to optimize at least one wavelength of the harmonics in order to maximize the measurement sensitivity of the measured target structure, any one of clauses 51 to 54 The harmonic generation radiation source described in the item.
56. The output radiation spectrum includes a plateau region having a plurality of the above-mentioned harmonic peaks with similar intensities, and a cutoff region in which the intensity largely falls for each successive harmonic peak, and the wavelength is controlled / optimized harmonics 54. A harmonic generating radiation source according to clause 54 or 55, wherein said at least one of the wave peaks is a plateau region harmonic peak.
57. The harmonic generating radiation source according to any one of clauses 51 to 56, wherein the intensity of the drive laser pulse is a long cycle and the number of cycles of the drive laser pulse is more than 15.
58. 57. A source for generating harmonics according to clause 57, wherein the number of cycles of the driving laser pulse is 15-30.

  [0086] In the text, although specific mention could be made of the use of a lithographic apparatus in IC manufacture, it will be appreciated that the lithographic apparatus described herein is for the manufacture of integrated optical systems, for magnetic domain memories And other applications such as flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc. Those skilled in the art will recognize that, in the context of such alternative applications, the terms "wafer" or "die" as used herein are synonymous with the more general terms "substrate" or "target portion", respectively. It can be considered that there is. The substrates referred to herein may be treated, for example, with a track (typically a tool for applying a resist layer to a substrate and developing the exposed resist), a metrology tool, and / or an inspection tool, before or after exposure. can do. Where applicable, those disclosed herein can be applied to such and other substrate processing tools. Furthermore, the term substrate as used herein can also refer to a substrate that already comprises multiple processed layers, as the substrate can be processed more than once, for example, to form a multilayer IC.

  [0087] Although specific reference could be made above to the use of the embodiments of the invention in the context of optical lithography, it will be appreciated that the invention has other applications, for example imprint lithography It can be used and is not limited to optical lithography where circumstances allow. In imprint lithography a topography in a patterning device defines the pattern to be formed on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is pulled away from the resist leaving a pattern in it after the resist is cured.

  [0088] The terms "radiation" and "beam" used in connection with a lithographic apparatus (eg having a wavelength of 365, 355. 248, 193, 157 or 126 nm or around them) Ultraviolet (UV) radiation and extreme ultraviolet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as all types of electromagnetic radiation, including particle beams such as ion beams or electron beams.

  [0089] The term "lens", where the context allows, may be any one of a variety of types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components, or It can point to the combination of those.

  [0090] The foregoing description of specific embodiments fully illustrates the general nature of the present invention, so that others may apply excessive knowledge by applying the knowledge within the skill of the art. Such specific embodiments can be modified and / or such specific embodiments can be adapted to various applications without experimentation and without departing from the general concepts of the present invention. Accordingly, such adaptations and modifications are intended to be within the spirit and scope of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the terminology or terms herein is for the purpose of description by way of example and not limitation, and the terms or technical terms herein are to be read in light of the teachings and guidance. It should be interpreted by the same person.

  The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims (15)

A method of measuring with an inspection device,
Setting one or more controllable characteristics of at least one drive laser pulse of the harmonic generating radiation source to control the output radiation spectrum of the illumination radiation supplied by the harmonic generating radiation source;
Illuminating a target structure with the illumination radiation;
Method including.   The method of claim 1, wherein the setting step comprises setting one or more controllable characteristics of a drive laser pulse such that the output radiation spectrum includes a plurality of discrete harmonic peaks.   The method of claim 2, wherein the bandwidth of each harmonic peak is narrower than 0.2 nm.   The method according to claim 2 or 3, wherein the spectral spacing between adjacent harmonic peaks is greater than the bandwidth of the harmonic peaks.   The method according to any one of claims 2 to 4, wherein the setting step comprises setting a central wavelength of the drive laser pulse to control at least one wavelength of the harmonic peak.   The setting step comprises setting the center wavelength of the drive laser pulse so as to optimize the wavelength of at least one of the harmonics in order to maximize the measurement sensitivity of the target structure. The method according to any one of 2 to 5.   The output radiation spectrum includes a plateau region having a plurality of the harmonic peaks having similar intensities, and a cutoff region in which the intensity largely falls for each successive harmonic peak, and the wavelength is controlled / optimized The method according to claim 5 or 6, wherein the at least one of the harmonic peaks to be generated is a harmonic peak in the plateau region.   The method according to any one of claims 1 to 7, wherein the intensity of the drive laser pulse is a long cycle and the setting step comprises setting the number of cycles of the drive laser pulse to be more than 15. Method described.   The method according to claim 8, wherein the setting step comprises setting the number of cycles of the drive laser pulse to 15-30.   The setting step may include setting a driving field of the harmonic generating radiation source by setting one or more controllable characteristics of a plurality of driving laser pulses at least partially including different central wavelengths. The method of claim 1.   11. The method of claim 10, wherein the drive laser pulse consists of three.   The setting step temporarily adapts the drive electric field of the harmonic generating radiation source so that the narrow band of the output radiation spectrum is enhanced and the remaining part of the output radiation spectrum is suppressed. The method according to claim 10, comprising setting the one or more controllable characteristics of the plurality of drive laser pulses. Detecting scattered radiation from the illumination of the target structure;
Determining overlay offsets between various layers of the target structure from the scattered radiation;
13. A method according to any one of the preceding claims, comprising   14. An inspection apparatus comprising a harmonic generating radiation source and operable to perform the method according to any one of the preceding claims.   A computer program product comprising machine readable instructions which, when run on a suitable processor, cause the processor to perform at least the setting steps of the method according to any one of the preceding claims.

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